Physical variables are acquired via sensors in an air system of an internal combustion engine and are used as input variables for calculating further modeled variables.
A method, for example, is known from the non-prepublished German patent publication DE 10 2010 002849 how air pressure upstream of a throttle valve can be calculated from an air mass flow which was ascertained. In the case of a diesel engine having a turbocharger, pressure in the exhaust gas tract can be calculated from the charge of the internal combustion engine, the quantity of fuel injected, the rotational speed of the turbocharger, a geometry factor of the turbocharger and the temperature in the exhaust gas tract.
Controllers as, for example, boost pressure or air mass controllers, are based on the aforementioned variables in order to ensure an optimal engine charge with regard to emissions, consumption etc. Error-prone sensors would therefore lead to an increase in harmful emissions and thus by law have to be diagnosed for malfunctions. In this case, it is required by law for the malfunction of an individual sensor to be reliably diagnosed. On the other hand, so-called double faults, i.e. the simultaneous malfunction of two or more sensors, do not have to be considered under the law. A diagnostic method can therefore proceed from the premise that maximally one sensor is defective.
Electrical diagnoses for waste cables, shorts to ground and power supply can, for example, be carried out. Furthermore, a plausibility check can be made of one sensor to a second sensor and if need be an offset compensation at a defined working point can be carried out. It is, for example, known from prior art how to check the plausibility of a boost pressure sensor and an exhaust gas back pressure sensor with an ambient pressure sensor when the engine is switched off. The working point thereby defined is, however, only one possible operating point of the sensor and in no way covers the full operating range. In addition, it is possible to check the plausibility of an air mass flow meter with the boost pressure sensor. No specific “pinpointing” can, however, occur using this procedural approach. That is to say it cannot be concluded whether the boost pressure sensor or the air mass flow meter is defective.
A diagnosis does not exist for the exhaust gas back pressure sensor over the full operating range of said exhaust gas back pressure sensor.